Question:

In what situations is substitution cypher easy to decode? Why is this?

Answer:

Substitution Cypher is relatively easy to decode as long as you have a large sample size of text. The larger of a sample size you have, the easier it is to decode for a number of reasons. For one, the more coverage over the alphabet you have, the easier it is to do a statistical analysis and guess which letters are more likely where. Next, the long words are easier to decode because they are not as common and can knock out quite a few more letters at once. Repetition of words will also assist in the discernment of which letters are what, so long as the sample size supports it. Again, statistical analysis and context sensitivity are important.